Performance of small, inexpensive electrochemical sensors for rapid and accurate measurement of free and total chlorine in aqueous solution have been impacted by interfering background currents arising from changes in potential of the working electrode which defines the limit of detection for such devices (≧0.1 ppm for chlorine). Such background currents may be measured in analyte-free solutions and subtracted from the current measured when analyte is present. However, the additional measurement increases the complexity of the sensor and increases the quantity of reagent required.
Flow-injection analysis allows the background current to decay to a minimum value under analyte-free conditions, at which time samples containing the analyte are injected into the background electrolyte solution and an amperometric signal is generated. Lower limits of detection of 5 ppb of chlorine have been reported using this detection method; however, controlled flow of fluids and analyte-free background solutions are required, making the system more complex. See, e.g., “A Miniaturized FIA System for the Determination of Residual Chlorine in Environmental Water Samples,” by Jiye Jin et al., Anal. Sci. 20, pages 205-207 (2004).
Use of surrogate analytes for improving the selectivity of the electrochemical process has been reported. In “Reaction with N,N-Diethyl-p-phenylenediamine: A Procedure for the Sensitive Square-Wave Voltammetric Detection of Chlorine” by E. Hugo Seymour et al., Electroanalysis 15, pgs. 689-694 (2003), the authors reported the reaction of N,N-diethyl-p-phenylenediamine (DPD) with chlorine in solution which, when combined with square-wave voltammetry, was found to improve the sensitivity of chlorine measurements at a bare, glassy carbon electrode. The reaction was found to overcome problems related to the direct reduction of chlorine at the electrode surface, and provides selectivity for the determination of chlorine in the presence of other halogen species and sulfide. The technique yields a detection limit of 0.45 ppm for chlorine as Cl2.
Self-assembled monolayers have been used for reduction of background and improving selectivity of electrochemical sensors. See, e.g., “A Signal Amplification Scheme for Ultrasensitive Amperometric Detection in Flowing Streams” by Phillip T. Radford et al., Anal. Chem. 71, pgs. 5101-5108 (1999), wherein the authors demonstrated that a self-assembled monolayer (SAM) formed on a gold electrode provides selective oxidation of a poorly hydrated and neutral ferrocene species over that of a strongly hydrated and charged ferrocyanide species.
Accordingly, it is an object of the present invention to provide an apparatus and method for rapidly measuring the concentrations of free chlorine and total chlorine in aqueous samples having low chlorine concentrations.
Another object of the present invention is to provide an apparatus and method for rapidly measuring the concentration of free chlorine and total chlorine in aqueous samples having low chlorine concentrations, and requiring small sample volumes.
Yet another object of the present invention is to provide an apparatus and method for rapidly measuring the concentration of free chlorine and total chlorine in aqueous samples having low chlorine concentrations, and requiring small reagent volumes.
Still another object of the present invention is to provide an apparatus and method for rapidly measuring the concentration of free chlorine and total chlorine in aqueous samples having low chlorine concentrations, and employing a single-use, disposable sensor.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.